I use both a rangefinder and a GPS. For most of my shots, I use just the GPS. But for Par-3's, shots where I have to carry a bunker or hazard, or there's a significant elevation change, then it's the Rangefinder all the way.

If I didn't already have both - I'd probably just go with a GPS. But since the slope feature came out, I really like having some idea about how much to compensate for -- I like having that new 'toy', probably because I'm kind of a gadget guy.
-- P.J.

What a great topic....
Well my go to was the Swing By Swing app.....But they have updated it, and took out the sharing of the live round of social media. I was able to share my round and friends were able to follow the scoring live. I have requested that they put that back in, so we will see.
I am with PJ on shooting the distance on par 3's with a range finder.
For my GPS App, I started using the VPAR app the last 4 rounds. Pretty accurate, and easy to use scorecard feature that will keep your 4somes score. Easy on the battery too.

Used to be my go-to was using the course markers(And knowing the course)......now I use the Shot Scope V2 on the course (Thanks again Bigg's Golf Talk for the win). It'll show me yardage to front,back and centre of the green.......and keeps my score and stats for when I get off the course.